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Abstract:

Provided is a high-performance pneumatic radial tire for passenger cars
which uses a hybrid cord composed of an aramid cord and a Nylon 66 cord
having different twist counts for the first twist and different twist
counts for the second twist, as a reinforcing belt, and thereby has
improved high speed durability and steering stability.

Claims:

1. An aramid-Nylon 66 hybrid cord comprising an aramid cord and a Nylon
66 cord doubled and twisted together, wherein the ratio of the elastic
modulus of the hybrid cord at an elongation ratio of 3% to 5% with
respect to the elastic modulus at an elongation ratio of 0.5% to 1% is
from 1 to 10.

2. The aramid-Nylon 66 hybrid cord according to claim 1, wherein the
aramid cord having a large initial elastic modulus has a twist count of
the first twist of 200 TPM to 500 TPM, and the Nylon 66 cord having a
small initial elastic modulus has a twist count of the first twist of 50
TPM to 200 TPM.

3. The aramid-Nylon 66 hybrid cord according to claim 2, wherein the
ratio of twist count of the aramid cord to the twist count of the Nylon
66 cord is larger than 1.

4. A pneumatic radial tire comprising the aramid-Nylon 66 hybrid cord
according to claim 1 as a reinforcing belt.

Description:

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a pneumatic tire for passenger
cars, which uses a hybrid cord reinforced by having a particular twist
count as a reinforcing belt, and more particularly, to a high-performance
radial tire in which high speed durability and steering stability are
enhanced by applying cords that have different second twist counts and
different first twist counts to reinforcing belts.

[0003] 2. Description of the Related Art

[0004] A large number of technologies for applying aramid fibers as tire
cord yarns have been disclosed in the field of tire manufacturing
industry (see Korean Patent Application Laid-Open Nos. 2009-0164609,
2009-0114131, 2010-0006195, and the like).

[0005] Furthermore, technologies related to hybrid cords that are composed
of aramid filaments and nylon filaments have also been disclosed in the
field of tire industry (see Korean Patent Application Laid-Open Nos.
1989-0012812 and 2006-012601; Japanese Patent Application Laid-Open No.
2009-132324; and the like).

[0006] Japanese Patent Application Laid-Open No. 2009-132324 describes a
composite fiber cord in which two strands of aramid filaments and one
strand of a Nylon 46 filament are subjected to first twist and second
twist in opposite directions such that the second twist coefficient of
the composite fiber cord is adjusted to 1700 to 2700, and thereby high
speed durability and flat spot resistance are enhanced.

[0007] In tires, belts constitute very important factors which disperse
stress from the ground surface during vehicle running and maintain the
tire footprint shape, and thereby affect the running performance, wear
and brake performance. Among these factors, a reinforcing belt plays the
role of suppressing the displacement of a steel belt during vehicle
running and thereby maintains the tire performance.

[0008] Principal characteristics required from such a reinforcing belt
include heat-resistant adhesive power, contractile force, modulus
retention ratios at normal temperature and high temperatures, and the
like. Among these characteristics, heat-resistant adhesive power is a
factor which exerts the greatest influence on the durability of tires,
because when a reinforcing belt that is located between the belts and the
tread, which undergoes the severest heat generation in the tire, is
subjected to aging under high temperature during running due to its
positional characteristics, the reinforcing belt must maintain the
adhesive power at a certain level. Contractile force has an effect of
preventing the phenomenon in which durability is deteriorated due to tire
expansion caused by an increase in temperature during running, through
the contraction of the reinforcing belt. A modulus is a factor which
affects the comfort of ride and steering stability, and a reinforcing
belt having a large modulus tends to reduce the change in the shape of a
steel belt that is caused by the force transmitted from the outside, and
thereby enhances steering stability. However, on the other hand, a
reinforcing belt having a large modulus tends to mitigate the comfort of
ride.

[0009] Tires for passenger cars make use of cords formed of polyethylene
terephthalate (PET), rayon, nylon and the like are used. A PET cord has
inferior heat-resistant adhesive power and has its modulus greatly
decreased at high temperatures, and thus, it is found difficult to apply
a PET cord as a reinforcing belt. Rayon has almost no contractile force,
has low fatigue resistance, and cannot support a steel belt. Therefore,
it is impossible to use rayon in tires. Lastly, in the case of nylon,
which is currently widely used, Nylon 66 is advantageous, compared with
Nylon 6, in that the heat-resistant adhesive power is excellent, and the
modulus decrement at high temperatures is low. Thus, it can be said that
Nylon 66 is a material for reinforcing belt that is currently most
frequently used all over the world. However, since the modulus which is
considered as the most fundamental characteristic of nylon is very low,
when nylon is applied to high performance radial tires for which there is
a high possibility that the internal temperature of the tire increases as
a result of high speed running, the durability performance is adversely
affected.

SUMMARY OF THE INVENTION

[0010] Accordingly, it is an object of the present invention to provide a
high performance radial tire which has enhanced high speed durability and
steering stability without any change in the comfort of ride, by applying
a hybrid cord obtained by combining an aramid cord and a Nylon 66 cord.

[0011] The radial tire according to the present invention can have its
high speed durability and steering stability enhanced by 10% to 15%
without compromising the comfort of ride, by using two first twists
having elastic moduli that are different from each other, and the
difference in the twist count of the respective first twists.

[0012] More particularly, by doubling and twisting cords having high
elastic module while maintaining the twist count of the cord having a
lower elastic modulus low at the time of the production of a hybrid cord,
the characteristics of the cord having a relatively lower elastic modulus
are caused to be exhibited during low speed running, and the
characteristics of the cord having a relatively higher elastic modulus
are caused to be exhibited during high speed running. Accordingly, at the
time of low speed running, a cord having excellent fatigue performance
such as nylon is applied so as to maintain the comfort of ride unchanged,
and at the time of high speed running, a cord having excellent heat
resistant performance such as aramid is applied so as to suppress tire
expansion caused by an increase in temperature inside the tire. Thereby,
high speed durability and steering stability are enhanced to or above a
certain level.

[0013] Embodiments of the present invention will be described in more
detail.

[0014] The present invention relates to a pneumatic tire for passenger
cars, which uses a hybrid cord obtained by doubling and twisting an
aramid cord and a Nylon 66 cord.

[0015] Since Nylon 66 is excellent in the heat resistant adhesive power,
contractile force, and the elastic modulus at a low temperature and under
a low load, Nylon 66 is used for the application in reinforcing belts for
tires and plays the role of maintaining the comfort of ride and steering
stability of the tire. However, it is considered that this is effective
only in the sections of low speed and general economic speed. As time
passes, passenger cars acquire improved performance, and accordingly,
there is an increasing demand for tire durability performance upon high
speed running. For this reason, when Nylon 66 is used alone, the shape
stability of the tire at the time of high speed running cannot be
secured. In order to maintain the elastic modulus at high temperatures,
the inventors of the present invention solved the problem by using a high
elastic modulus material such as aramid in combination with Nylon 66. At
this time, in order to exhibit the characteristics of aramid and Nylon
66, the twist counts of the first twist were made different from each
other such that the twist count of a Nylon 66 cord was adjusted to 50 TPM
(twists per meter) to 200 TPM, while the twist count of an aramid cord
was adjusted to 200 TPM to 500 TPM. The second twist was adjusted such
that the twist count of the Nylon 66 cord would be always lower than the
twist count of aramid cord. When the twist count of the second twist of
the aramid cord which has a relatively lower elongation ratio is kept
lower than the twist count of the second twist of the Nylon 66 cord, the
characteristics of nylon at the time of low speed running cannot be
realized. When the twist count of the first twist of the aramid cord is
adjusted to 200 TPM or less, fatigue performance of the tire is
decreased. On the contrary, when the twist count of the first twist of
the aramid cord is adjusted to 500 TPM or more, strength is decreased due
to the friction between filaments, and the durability performance of the
tire is deteriorated.

[0016] Furthermore, the aramid hybrid cord should be such that the ratio
of the elastic modulus at an elongation ratio of 3% to 5% with respect to
the elastic modulus at an elongation of 0.5% to 1% is in the range of
from 1 to 10. If the ratio of elastic modulus is less than 1, the shape
stability of the tire according to the characteristics of nylon upon high
speed running becomes poor, and the running performance of the tire is
deteriorated. If the ratio of elastic modulus is greater than 10, there
is a defect that the high elastic modulus characteristic of the aramid
cord is disadvantageous in the forming of the tire, and the comfort of
ride during running is deteriorated [see Table 1 and FIG. 1].

[0017] The evaluation of the pneumatic radial tire according to the
present invention was carried out by using a radial tire in which the
original Nylon 66 reinforcing belt was replaced by an aramid-Nylon 66
hybrid cord-reinforced belt, and no change was made on the design factors
other than the alteration of the reinforcing belt.

[0018] The high performance radial tire according to the present invention
exhibits improved high speed durability and improved steering stability
without any change in the comfort of ride, by applying a hybrid cord
obtained by combining an aramid cord and a Nylon 66 cord as a reinforcing
belt.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 is a graph illustrating the relationships between modulus
and strain in an aramid cord alone, a hybrid cord of an aramid cord and a
Nylon 66 cord, and a Nylon 66 cord alone.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, the present invention will be described in detail by
way of Examples, but the present invention is not intended to be limited
to the Examples.

[0022] 1) The fracture strength and fracture elongation were measured
according to ASTM D885.

[0023] 2) The heat resistant adhesive power were measured according to
H-Test ASTM D4776, and the adhesive power after aging the cord for 24
hours at 100° C. was measured.

[0024] According to the results of the above Table 1, as can be seen from
the elastic modulus at various elongation ratios of the cord, the elastic
moduli at an elongation of 0.5% of Examples 1 and 2 according to the
present invention were lower than the elastic modulus of Comparative
Example 1, and it can be seen that as the elongation ratio increases from
0.5% to 5.0%, the elastic modulus of the hybrid cord increases
proportionally to the elongation ratio. In the case of the hybrid cords,
it can be seen that Example 2 which had a higher twist count of the
second twist exhibits the low elastic modulus characteristic of Nylon 66
at a low elongation ratio.

[0025] Tires in which a Nylon 66 hybrid cord mixed with an aramid cord,
the two having different twist counts, was applied to the reinforcing
belt, were produced according to Table 2.

[0026] An evaluation of the high speed running durability and the steering
stability of the tires thus produced, and an evaluation of the running
safety after high speed running were carried out as described below, and
the results are presented in the following Table 3.

[0027] 1) Evaluation of High Speed Running Durability

[0028] The high speed running durability was evaluated according to
ECE-R30, which is a high speed running durability test method.

[0029] 2) Evaluation of Steering Stability

[0030] A vehicle equipped with the tire was steered by a professional test
driver who can perceive the change in steering stability, and thus the
extent of steering stability is examined. Here, the initial steering
stability means the steering stability of the early stage of tire
installation, and the evaluation of steering stability after high speed
running means the steering stability in the state that a test tire has
been preheated after running for 30 minutes at a velocity of 230 km/hr.

[0032] As can be seen from the results of Table 3, in Application Example
1 and Application Example 2 in which a reinforcing belt cord reinforced
with a hybrid cord of an aramid cord and a Nylon 66 cord, the two having
different twist counts, was applied in the tire, the high speed running
durability and the steering stability upon low speed running and high
speed running were all excellent. On the other hand, the tire which used
a conventional Nylon 66 cord as a reinforcing belt cord exhibits
relatively poor results in the high speed running durability and the
steering stability upon low speed running and high speed running.

[0033] As discussed above, the reinforcing belt according to the present
invention reinforced with a hybrid cord of an aramid cord and a Nylon 66
cord, the two having different twist counts, exhibits excellent results
in the high speed durability and steering stability without compromising
the comfort of drive.